Since NCStudio does not do PWM or voltage modulation... it has 3 wires going to the spindle that are simply high/low state (on/off).
The spindle inverter has 3 ports for 'programmable control'. So SaleCNC simply setup the 3 ports on the spindle for RPM speeds of 5k on port1, 10k on port2, and 24k on port3.
You will see on an NCStudio breakout illustration 3 pins marked "H M L".... High, Medium & Low... NCStudio was actually designed to operate this way
This does not mean it's permanently stuck this way though.... you can use the LCD control panel to reprogram the HML speed settings, or you can disable the 3 input ports and just manually set the RPM from that LCD panel before machining.